Class-ab stabilization

ABSTRACT

Aspects of the description provide for a circuit. In some examples, the circuit includes a input pair of transistors, a bias transistor having a bias transistor gate, a bias transistor drain, and a bias transistor source, the bias transistor drain coupled to the input pair of transistors and the bias transistor source coupled to ground, and a resistor coupled between the bias transistor gate and the input pair of transistors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/470,985, filed Sep. 9, 2021, which claims priority to IndiaProvisional Patent Application No. 202041051297, filed Nov. 25, 2020,entitled “Stabilization of Class-AB Loop For Low Voltage Class-ABArchitecture,” which applications are hereby incorporated herein byreference.

BACKGROUND

In certain circuit topologies, such as amplifiers, feedback isimplemented as an aspect of controlling operation of the circuit. Thatfeedback may be stabilized to prevent, or mitigate, the aspects of thefeedback from adversely affecting performance of the circuit.

SUMMARY

Some aspects of the description provide for an amplifier. In someexamples, the amplifier includes a first transistor having a firsttransistor source and a first transistor drain, and a second transistorhaving a second transistor gate, a second transistor source, and asecond transistor drain. The first transistor drain is coupled to anoutput of the amplifier, the second transistor source is coupled to thefirst transistor source, and the second transistor drain is coupled tothe output of the amplifier. The feedback transistor has a feedbacktransistor gate, a feedback transistor source, and a feedback transistordrain. The feedback transistor source is coupled to ground and thefeedback transistor drain is coupled to the second transistor gate. Theresistor is coupled between the second transistor gate and the feedbacktransistor gate.

Some aspects of the description provide for a circuit. In some examples,the circuit includes a first transistor and a negative feedback circuit.The first transistor has a first transistor gate and a first transistordrain. The first transistor drain is coupled to an output of thecircuit. The negative feedback circuit is coupled to the firsttransistor gate. The negative feedback circuit is adapted to bias thefirst transistor to provide negative feedback to the circuit. Thenegative feedback circuit includes a second transistor, a resistor, athird transistor, and a current minor. The second transistor has asecond transistor gate, a second transistor source, and a secondtransistor drain. The second transistor source is adapted to be coupledto ground and the second transistor drain is coupled to the firsttransistor gate. The resistor is coupled between the first transistorgate and the second transistor gate. The third transistor has a thirdtransistor gate, a third transistor source, and a third transistordrain. The third transistor gate is coupled to the output of the circuitand the third transistor drain is coupled to the first transistor drain.The fourth transistor has a fourth transistor gate, a fourth transistorsource, and a fourth transistor drain. The fourth transistor gate iscoupled to the output of the circuit and the third transistor source isadapted to be coupled to ground. The current mirror is coupled betweenthe third transistor source and the fourth transistor drain.

Some aspects of the description provide for a circuit. In some examples,the circuit includes an input pair of transistors, a bias transistorhaving a bias transistor gate, a bias transistor drain, and a biastransistor source, the bias transistor drain coupled to the input pairof transistors and the bias transistor source adapted to be coupled toground, and a resistor coupled between the bias transistor gate and theinput pair of transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows a circuit diagram of an amplifier in accordance withvarious examples.

FIG. 2 shows a frequency response diagram in accordance with variousexamples.

FIG. 3 shows a diagram of frequency response locations of zeros andpoles in accordance with various examples.

FIG. 4 shows a circuit diagram of an amplifier in accordance withvarious examples.

DETAILED DESCRIPTION

In some circuit topologies, such as class-AB amplifiers, or otheramplifier structures, negative feedback is implemented. The negativefeedback may provide for stability in the circuit, such as to reducetime domain ringing and frequency domain peaking. However, the negativefeedback may also introduce a pole into a frequency response of theamplifier. If the pole is near, or within, a unity-gain bandwidth of theamplifier, stability of the amplifier may be adversely affected. In atleast some examples, existence of the pole is inherent to a negativefeedback loop of the amplifier, such as being inherent to a component ofthe negative feedback loop (e.g., such as a parasitic capacitance of atransistor). In this way, introduction of the pole into the frequencyresponse of the amplifier may be unavoidable when a negative feedbackloop is also implemented.

Aspects of this description provide for increasing stability of anamplifier having a negative feedback loop. In some examples, thestability is increased by adding an additional component, or components,into the negative feedback loop to shift a position of the pole. In someexamples, the stability is increased by an additional component, orcomponents, into the negative feedback loop to at least partiallymitigate or negate the pole. In some examples, the stability isincreased by dividing, separating, and/or replicating components of theamplifier, such as a by splitting an input transistor pair of theamplifier.

FIG. 1 shows a circuit diagram of an amplifier 100 in accordance withvarious examples. In at least some examples, the amplifier 100 is aclass-AB operational amplifier. The amplifier 100 may receive an inputsignal at an input voltage (VIN) node 102 and provide an output signalat an output voltage (VOUT) node 104. The node 104 may correspond to apositive input of a differential input pair of the amplifier 100. In atleast some examples, the amplifier 100 also includes a feedback voltage(VFB) node 106. The node 106 may correspond to a negative input of thedifferential input pair of the amplifier 100. A feedback signal may bereceived at the VFB node 106 and may be based at least partially onvalue of VOUT. Although not shown, in some examples the VFB node 106 iscoupled to the VOUT node 104. For example, the VFB node 106 may becoupled to the VOUT node 104 through one or more intervening components,or without intervening components. In at least some examples, theamplifier 100 includes a transistor 108, a transistor 110, a transistor112, a transistor 114, a transistor 116, a transistor 118, a transistor120, a transistor 122, a transistor 124, a transistor 126, a transistor128, a transistor 130, a transistor 132, a transistor 134, a transistor136, a transistor 138, a transistor 140, a transistor 142, a transistor144, a transistor 146, a transistor 148, a capacitor 150, a capacitor152, a capacitor 154, a resistor 156, a transistor 178, a transistor180, and a transistor 182. In at least some examples, relationships mayexist between the sizes (e.g., widths or width-to-length ratios) of someof the components of the amplifier 100. For example, the transistor 136may have a size that is N times larger than a size of the transistor182, where N is any suitable positive, real-number value. Similarly, thetransistor 148 may have a size that is M times larger than a size of thetransistor 144, where M is any suitable positive, real-number value. Thetransistor 138 and the transistor 140 may have an approximately samesize, with the transistor 142 having a size that is K times larger thanthe size of the transistor 140 and the transistor 146 having a size thatis P times larger than the size of the transistor 138, where K and P areeach any suitable positive, real-number values.

In an example architecture of the amplifier 100, the transistor 108 hasa gate coupled to the VIN node 102, a source coupled to a node 158, anda drain coupled to a node 160. The transistor 110 has a gate coupled tothe VFB node 106, a source coupled to the node 158, and a drain coupledto a node 162. The transistor 112 has a drain coupled to a node 164, asource, and a gate. The transistor 114 has a drain coupled to the sourceof the transistor 112, a source coupled to the node 158, and a gate. Thetransistor 116 has a source coupled to the node 164, a gate coupled tothe gate of the transistor 112, and a drain. The transistor 118 has asource coupled to the drain of the transistor 116, a gate coupled to thegate of the transistor 114, and a drain coupled to a node 166. Thetransistor 120 has a drain coupled to the node 166, a source coupled tothe node 160, and a gate. The transistor 122 has a gate coupled to thenode 166, a drain coupled to the node 160, and a source coupled to anode 168. The transistor 124 has a gate coupled to the node 166, a draincoupled to the node 162, and a source coupled to the node 168. In atleast some examples, the transistor 122 and the transistor 124 form acurrent mirror between the node 160 and the node 162. In at least someexamples, the amplifier 100 may include a negative feedback loop. Thenegative feedback loop may attempt to control an amount of currentprovided to the VOUT node 104 to cause VFB to approximately equal VIN.The negative feedback loop may include the transistors 126-134, thecapacitor 152, the capacitor 154, the transistors 138-144, the capacitor150, and the resistor 156.

The transistor 126 has a gate coupled to the gate of the transistor 120,a drain coupled to a node 170, and a source coupled to the node 162. Thetransistor 128 has a drain coupled to a node 172, a gate coupled to anode 174, and a source coupled to the node 162. The transistor 126 andthe transistor 128 may together be an input transistor pair (which mayalso be referred to as a differential input pair or a class-AB inputpair) of the amplifier 100. The transistor 130 has a gate coupled to thegate of the transistor 114, a drain coupled to the node 170, and asource coupled to a node 176. The transistor 132 has a gate coupled tothe gate of the transistor 112, a drain coupled to the node 176, and asource coupled to the node 164. The transistor 134 has a gate coupled tothe gate of the transistor 114, a drain coupled to the node 172, and asource coupled to the node 176.

The transistor 136 has a gate coupled, or adapted to be coupled, throughthe resistor 156 to the node 174 and coupled, or adapted to be coupled,through the capacitor 150 to the node 168. The transistor 136 also has adrain coupled to the node 174 and a source coupled to the node 168. Thetransistor 138 has a gate coupled to the node 170, a drain coupled tothe node 174, and a source. The transistor 140 has a gate coupled to anode 177, a drain coupled to the source of the transistor 138, and asource coupled to the node 164. The transistor 142 has a gate coupled tothe node 177, a drain coupled to the node 177, and a source coupled tothe node 164. In at least some examples, the transistor 140 and thetransistor 142 form a current minor between the node 177 and the sourceof the transistor 138.

The transistor 144 has a gate coupled to the node 172, a drain coupledto the node 177, and a source coupled to the node 168. The transistor146 has a gate coupled to the node 170, a drain coupled to the VOUT node104, and a source coupled to the node 164. The transistor 148 has a gatecoupled to the node 172, a drain coupled to the VOUT node 104, and asource coupled to the node 168. The VOUT node 104 is coupled, or isadapted to be coupled, to the node 170 via the capacitor 152. The VOUTnode 104 is coupled, or is adapted to be coupled, to the node 172 viathe capacitor 154. The transistor 178 has a gate coupled to the gate ofthe transistor 112, a source coupled to the node 164, and a drain. Thetransistor 180 has a gate coupled to the gate of the transistor 114, asource coupled to the drain of the transistor 178, and a drain. Thetransistor 182 has a gate coupled to the gate of the transistor 120, adrain coupled to the gate of the transistor 120, and a source coupled tothe node 168.

In at least some examples, the node 164 is adapted to couple to avoltage supply (not shown) to receive a first supply voltage at the node164. In at least some examples, the node 164 is adapted to couple to avoltage supply (not shown) to receive a second supply voltage at thenode 168. In at least some examples, the second supply voltage is aground voltage potential. In other examples, the second supply voltagemay have a non-zero value that is greater than, or less than, the firstsupply voltage. In some examples, the amplifier 100 may be adapted tocouple to separate voltage supplies at the node 164 and the node 168,while in other examples the amplifier 100 may be adapted to couple tothe same voltage supply at the node 164 and the node 168 (e.g., such aspositive and negative nodes or terminals of the same voltage supply).

In an example of operation of the amplifier 100, VIN is received at theVIN node 102 having a value greater than a value of VFB as received atthe VFB node 106. In such an example, the amplifier 100 attempts toprovide an amount of current to the VOUT node 104 to cause VOUT toincrease in value an amount sufficient to cause VFB to increase in valueto approximately equal the value of VIN. Conversely, VIN may be receivedat the VIN node 102 having a value lesser than a value of VFB asreceived at the VFB node 106. In such an example, the amplifier 100attempts to provide an amount of current to the VOUT node 104 to causeVOUT to decrease in value an amount sufficient to cause VFB to decreasein value to approximately equal the value of VIN. Operation of theamplifier 100 may be substantially similar in each of these operationalcircumstances. Therefore, the example in which VIN is greater in valuethan VFB is used herein to describe operation of the amplifier 100. Forexample, a tail current (Itail) flows through the transistor 112 and thetransistor 114 to the node 158. Responsive to receipt of VIN having avalue greater than VFB, an amount of current flows from the node 158,through the transistor 110, to the node 162. Current also flows from thenode 168 through the transistor 122 and the transistor 108 to the node158 such that a sum of current flowing into and out of the node 158 isapproximately zero. The current flowing through the transistor 122 ismirrored to also flow through the transistor 124 to the node 162,summing at the node 162 with the current flowing through the transistor110. The current flowing through the node 162 flows through thetransistor 126 and the transistor 128 to the node 170 and the node 172,respectively. The current that flows through the transistor 126 and thetransistor 128 may be determined according to a ratio of widths of thetransistor 126 to the transistor 128. The current flowing through thenode 170 flows through the capacitor 152 to the VOUT node 104 and thecurrent flowing through the node 172 flows through the capacitor 154 tothe VOUT node 104, each to charge the VOUT node 104. In at least someexamples, the capacitor 152 and the capacitor 154 are Millercompensation capacitors in the amplifier 100.

In at least some examples, the transistor 146 sources current to theVOUT node 104 responsive to a load (not shown) coupled to the VOUT node104 drawing or sinking current. To source current to the VOUT node 104,the amplifier 100 decreases the potential at the gate of the transistor146 (e.g., the node 170), increasing conductance of the transistor 146.Similarly, the potential at the gate of the transistor 138 decreases,causing the transistor 138 to operate in a saturation region, or mode,of operation in which the transistor 138 operates as a cascodetransistor. Responsive to the transistor 138 operating as a cascodetransistor, a current flowing through the transistor 138 is determinedbased on a current flowing through the transistor 140, which may bedetermined based on the transistor 142 according to the current mirrorformed by the transistor 140 and the transistor 142. In at least someexamples, responsive to a voltage provided at the node 172 being athreshold amount greater than a voltage provided at the node 168, asense current (Isns) flows through the transistor 144 and therefore thetransistor 142, and Isns is mirrored to flow through the transistor 140as a feedback current (Ifb).

Ifb charges a parasitic capacitance provided at the node 174, causing apotential provided at the node 174 to increase. Responsive to thepotential provided at the node 174 increasing, the potential provided atthe node 172 may decrease and the potential provided at the node 170 mayincrease, such as via operation of the transistor 128 and the transistor126, respectively. Responsive to the potential provided at the node 172decreasing, the potential provided at the gate of the transistor 144decreases and Isns decreases. Responsive to the decrease in currentflowing through the transistor 142, the current mirrored to thetransistor 140 and flowing through the transistor 138 decreases,decreasing a potential provided at the node 174. Responsive to thepotential provided at the node 174 decreasing, the potential provided atthe node 172 may increase and the potential provided at the node 170 maydecrease, such as via operation of the transistor 128 and the transistor126, respectively. Responsive to the potential provided at the node 172increasing, the potential provided at the gate of the transistor 144increases and Isns increases. Responsive to the increase in currentflowing through the transistor 142, Ifb increases, again increasing apotential provided at the node 174. Similarly, as the potential providedat the node 170 increases, the current through transistor 138 tends todecrease, in turn decreasing the potential provided at the node 174. Inthis way, the amplifier 100 includes negative feedback that seeks tocontrol a potential provided at the node 174 to approximately equal apotential provided at the gate of the transistor 120.

In at least some examples, responsive to the potential provided at thenode 174 approximately equaling the potential provided at the gate ofthe transistor 120, Isns is approximately equal to a current flowingthrough the transistor 136, which is approximately equal to a currentflowing through the transistor 182. In such an example, with theamplifier 100 sourcing a load current at the VOUT node 104, a currentflowing through the transistor 148 may be approximately equal to a realnumber multiple of the current flowing through the transistor 182, wherethe real number is equal to a ratio of widths of the transistor 136 tothe transistor 182.

In at least some examples, rather than sourcing current to the VOUT node104, the amplifier 100 (e.g., at least via the transistor 148) sinkscurrent from the VOUT node 104 responsive to a load (not shown) coupledto the VOUT node 104 providing, or sourcing, current. To sink currentfrom the VOUT node 104, the amplifier 100 increases the potential at thegate of the transistor 148 (e.g., the node 172), increasing conductanceof the transistor 148. Similarly, the potential at the gate of thetransistor 144 increases, increasing Isns and decreasing a potentialprovided at the node 177. The decreased potential causes the transistor140 to operate in a linear region, or mode, of operation in which thetransistor 140 operates as a switch. Responsive to the transistor 140operating as a switch, Ifb becomes determined based on a current flowingthrough the transistor 146, such as via a current minor formed by thetransistor 138 and the transistor 146.

Ifb charges the parasitic capacitance provided at the node 174, causinga potential provided at the node 174 to increase. Responsive to thepotential provided at the node 174 increasing, the potential provided atthe node 172 may decrease and the potential provided at the node 170 mayincrease, such as via operation of the transistor 128 and the transistor126, respectively. As the potential provided at the node 170 increases,the current through transistor 138 tends to decrease, in turn decreasingthe potential provided at the node 174. Responsive to the potentialprovided at the node 172 decreasing, the potential provided at the gateof the transistor 144 decreases and a current flowing through thetransistor 144 (and transistor 142) decreases. Responsive to thedecrease in current flowing through the transistor 142, the currentmirrored to the transistor 140 and flowing through the transistor 138decreases, decreasing a potential provided at the node 174. Responsiveto the potential provided at the node 174 decreasing, the potentialprovided at the node 172 may increase and the potential provided at thenode 170 may decrease, such as via operation of the transistor 128 andthe transistor 126, respectively. Responsive to the potential providedat the node 172 increasing, the potential provided at the gate of thetransistor 144 increases and a current flowing through the transistor144 (and transistor 142) increases. Responsive to the increase incurrent flowing through the transistor 142, the current mirrored to thetransistor 140 and flowing through the transistor 138 increases, againincreasing a potential provided at the node 174. In this way, theamplifier 100 includes negative feedback that seeks to control apotential provided at the node 174 to approximately equal a potentialprovided at the gate of the transistor 120.

In at least some examples, responsive to the potential provided at thenode 174 approximately equaling the potential provided at the gate ofthe transistor 120, a current flowing through the transistor 144 isapproximately equal to a current flowing through the transistor 136,which is approximately equal to a current flowing through the transistor182. In such an example, with the amplifier 100 sinking a load currentat the VOUT node 104, a current flowing through the transistor 146 maybe approximately equal to a real number multiple of the current flowingthrough the transistor 182, where the real number is equal to a ratio ofwidths of the transistor 136 to the transistor 182.

The transistor 136 may bias the transistor 128. Therefore, to providefor proper operation of the amplifier 100 a gate-to-source voltage (Vgs)of the transistor 136 may be greater than, or equal to, Vgs of thetransistor 128 plus a drain-to-source voltage (Vds) of the transistor124. This Vgs of the transistor 136 may be comparatively large, leadingto the transistor 136 being a weak transistor that has a parasiticcapacitance (Cpara) that is comparatively large (e.g., such as about 500femtofarads). The comparatively large Cpara may cause the transistor 136to be slow to respond to signal transients, which may cause theamplifier 100 to also be slow to respond to signal transients. Forexample, the delay may be caused by the location of a pole in afrequency response of the transistor 136, which may have a value ofapproximately G136/Cpara, wherein G136 is a transconductance of thetransistor 136. If the pole is within a unity-gain bandwidth of theamplifier 100, the delay may contribute to ringing or signal oscillationbeing provided at the VOUT node 104.

To mitigate effects of this pole, in some examples, the resistor 156 iscoupled between the node 174 and the gate of the transistor 136 tointroduce a zero in the frequency response of the transistor 136. Thismay effectively split Cpara into two separate, approximately equalvalue, parasitic capacitances—Cpara1 experienced between the drain ofthe transistor 136 (e.g., the node 174) and the node 168 and Cpara2experienced between the gate of the transistor 136 and the node 168. Insuch an example, the resistor 156 may be selected to have a resistancevalue of approximately 1/G136. However, the resistor 156 alone may notmitigate the effects of the pole, in some operational circumstances.Therefore, in some examples, the capacitor 150 may be coupled betweenthe gate of the transistor 136 and the node 168. In at least someexamples, a capacitance of the capacitor 150 may be selected such that acapacitance between the gate of the transistor 136 and the node 168 isincreased. In some examples, resistance of the resistor 156 (R156) andcapacitance of the capacitor 150 (C158) are selected according to thefollowing equation 1, in which co is the unity-gain bandwidth of theamplifier 100 and a remainder of the variables are as defined above.

$\begin{matrix}{\frac{G136}{\left( {{C158} - {{Cpara}1} + {{Cpara}2}} \right)} < \frac{1/R156}{\left( {{C158} + {{Cpara}2}} \right)} < {G136*R158*\omega}} & (1)\end{matrix}$

In at least some examples, the resistor 156 and the capacitor 150 maycompensate for instability in the amplifier 100, resulting in increasedperformance of the amplifier 100. At least one example of the increasedperformance is a reduction in ringing or oscillation in a signalprovided at the VOUT node 104.

While the amplifier 100 is described herein as having feedback appliedat an n-type side of the amplifier 100 (e.g., via the transistor 126 andthe transistor 128), the feedback may instead be applied at a p-typeside of the amplifier 100 (e.g., via the transistor 130 and thetransistor 134). Operation of the amplifier 100 may be substantiallysimilar in such an implementation, thus description of such animplementation is not included herein.

FIG. 2 shows a frequency response diagram 200 in accordance with variousexamples. In at least some examples, the diagram 200 may be a Bode plotor Bode diagram that is at least partially representative of a frequencyresponse in the amplifier 100 of FIG. 1 . Accordingly, reference may bemade to aspects of FIG. 1 in describing the diagram 200. As shown in thediagram 200, a horizontal axis may be representative of ω in units ofmega radians per second (Mrad/s) and a vertical axis may berepresentative of a loop gain of the amplifier 100 in units of decibels(dB). The diagram 200 assumes values of 250 femtofarads for Cpara1 andCpara2.

As shown in the diagram 200, ideal performance of the amplifier 100 maybe linear in nature, having a ω (e.g., ω_ideal) of about 28 Mrad/s. Afrequency response of the amplifier 100 with ideal performance isdesignated as element 202. Due to process variation in components of theamplifier 100, operational characteristics, environmental effects,circuit instability, signal noise, and/or other factors, idealperformance may not be achievable. Instead, a non-compensatedperformance (e.g., “normal” performance) of the amplifier 100 may have aω of about 15 Mrad/s, with a frequency response pole at about 8 Mrad/s,above which ω decreases rapidly in value. A frequency response of thenon-compensated amplifier 100 is designated as element 204. However, ifcompensated by the resistor 156 and the capacitor 150, as describedabove, performance of the amplifier 100 may be increased. For example,for values of R156 approximately equal to 100 kiloohms and C158approximately equal to 2 picofarads (pF), the amplifier 100 may have a ω(e.g., ω_actual) of about 11 Mrad/s, with a frequency response poles atabout 1.8 and 40 Mrad/s, as well as a frequency response zero at about4.5 Mrad/s. A frequency response of the amplifier 100 includingcompensation is designated as element 206. The pole and zero locationsin the frequency response of the amplifier 100 are also shown in FIG. 3, which shows a diagram 300 of frequency response locations of zeros andpoles in the amplifier 100, in accordance with various examples, and asdescribed above with reference to the diagram 200. In the diagram 300, apole is represented as an X and a zero is represented as a solid dot(e.g., ⋅).

FIG. 4 shows a circuit diagram of an amplifier 400 in accordance withvarious examples. In at least some examples, at least some components ofthe amplifier 400 may be the same or substantially similar to those ofthe amplifier 100, and therefore are not described again with referenceto the amplifier 400. In at least some examples, the amplifier 400 is aclass-AB operational amplifier. The amplifier 400 may include acapacitor 184, a capacitor 186, a transistor 188, and a transistor 190.

In an example architecture of the amplifier 400, the capacitor 184 iscoupled between the VOUT node 104 and the node 176 and the capacitor 186is coupled between the VOUT node 104 and the node 162. Thus, thecapacitor 184 and the capacitor 186 may provide cascode compensation. Inat least some examples, the cascode compensation may provide increaseperformance with regard to power consumption and/or bandwidth incomparison to Miller compensation. In at least some examples, a ratio ofcapacitance of the capacitor 184 and the capacitor 186 to the capacitor152 and the capacitor 154, respectively, may be about 3:1, about 4:1, orany other suitable ratio. The capacitor 184 and the capacitor 186 mayoperate as common-mode capacitors that do not alter ω of the amplifier400. However, their presence may cause a pole in the frequency responseof the amplifier 400 to shift from outside of ω to within ω, creatinginstability as described above.

To mitigate the adverse effects of the cascode compensation on stabilityof the amplifier 400, the transistor 126 and the transistor 128 may beimplemented as split-pair transistors, coupled in parallel with thetransistor 188 and the transistor 190, respectively. The transistor 188and the transistor 190 may have a size ratio to the transistor 126 andthe transistor 128, respectively, of about (K−1):1, where K is anysuitable integer greater than or equal to 2. In at least some examples,implementing the transistor 126 and the transistor 128 as split-pairtransistors in this manner may reduce ω to a value of approximately

$\frac{\omega}{K},$

which may cause the pole shifted in the frequency response of theamplifier 400 by the cascode compensation to lie outside

$\frac{\omega}{K}.$

In this way, the split-pair transistors improve stability of theamplifier 400. In at least some examples, the split-pair transistors maybe implemented without including the cascode capacitance (e.g., theamplifier 400 may include the transistor 188 and the transistor 190 butnot include the capacitor 184 or the capacitor 186). For example, thesplit-pair transistors may be implemented to reduce a bandwidth of theamplifier 400. In some examples, the resistor 156 and/or the capacitor150 may be omitted when the split-pair transistors are implemented.

The term “couple” is used throughout the specification. The term maycover connections, communications, or signal paths that enable afunctional relationship consistent with this description. For example,if device A provides a signal to control device B to perform an action,in a first example device A is coupled to device B, or in a secondexample device A is coupled to device B through intervening component Cif intervening component C does not substantially alter the functionalrelationship between device A and device B such that device B iscontrolled by device A via the control signal provided by device A.

A device that is “configured to” perform a task or function may beconfigured (e.g., programmed and/or hardwired) at a time ofmanufacturing by a manufacturer to perform the function and/or may beconfigurable (or re-configurable) by a user after manufacturing toperform the function and/or other additional or alternative functions.The configuring may be through firmware and/or software programming ofthe device, through a construction and/or layout of hardware componentsand interconnections of the device, or a combination thereof.

A circuit or device that is described herein as including certaincomponents may instead be adapted to be coupled to those components toform the described circuitry or device. For example, a structuredescribed as including one or more semiconductor elements (such astransistors), one or more passive elements (such as resistors,capacitors, and/or inductors), and/or one or more sources (such asvoltage and/or current sources) may instead include only thesemiconductor elements within a single physical device (e.g., asemiconductor die and/or integrated circuit (IC) package) and may beadapted to be coupled to at least some of the passive elements and/orthe sources to form the described structure either at a time ofmanufacture or after a time of manufacture, for example, by an end-userand/or a third-party.

While certain components may be described herein as being of aparticular process technology, these components may be exchanged forcomponents of other process technologies. Circuits described herein arereconfigurable to include the replaced components to providefunctionality at least partially similar to functionality availableprior to the component replacement. Components shown as resistors,unless otherwise stated, are generally representative of any one or moreelements coupled in series and/or parallel to provide an amount ofimpedance represented by the shown resistor. For example, a resistor orcapacitor shown and described herein as a single component may insteadbe multiple resistors or capacitor, respectively, coupled in parallelbetween the same nodes. For example, a resistor or capacitor shown anddescribed herein as a single component may instead be multiple resistorsor capacitor, respectively, coupled in series between the same two nodesas the single resistor or capacitor.

Uses of the phrase “ground voltage potential” in the foregoingdescription include a chassis ground, an Earth ground, a floatingground, a virtual ground, a digital ground, a common ground, and/or anyother form of ground connection applicable to, or suitable for, theteachings of this description. Unless otherwise stated, “about,”“approximately,” or “substantially” preceding a value means +/−10percent of the stated value. Modifications are possible in the describedexamples, and other examples are possible within the scope of theclaims.

What is claimed is:
 1. An amplifier comprising: an output terminal; areference terminal; a first transistor having first and second currentpath terminals, wherein the second current path terminal of the firsttransistor is coupled to the output terminal; a second transistor havingfirst and second current path terminals, and a control terminal, whereinthe first current path terminal of the second transistor is coupled tothe first current path terminal of the first transistor, and wherein thesecond current path terminal of the second transistor is coupled to theoutput terminal; a third transistor having first and second current pathterminals, and a control terminal, wherein the first current pathterminal of the third transistor is coupled to the reference terminal,and wherein the second current path terminal of the third transistor iscoupled to the control terminal of the second transistor; and a resistorcoupled to the control terminal of the second transistor and to thecontrol terminal of the third transistor.
 2. The amplifier of claim 1,wherein the resistor is coupled between the control terminal of thesecond transistor and the control terminal of the third transistor. 3.The amplifier of claim 1, further comprising a capacitor coupled betweenthe resistor and the reference terminal.
 4. The amplifier of claim 3,wherein an intermediate node coupled between the resistor and thecapacitor is directly connected to the control terminal of the thirdtransistor.
 5. The amplifier of claim 3, wherein the resistor has aresistance selected according to:${\frac{G}{\left( {C + {{Cp}1} + {{Cp}2}} \right)} < \frac{1/R}{\left( {C + {{Cp}2}} \right)} < {G*R}},$wherein G represents a transconductance of the third transistor, Cp1represents a first parasitic capacitance between the first and secondcurrent path terminals of the third transistor, Cp2 represents a secondparasitic capacitance between the control terminal of the thirdtransistor the first current path terminal of the third transistorsource, C represents a capacitance of the capacitor, and R representsthe resistance of the resistor.
 6. The amplifier of claim 1, furthercomprising a fourth transistor having first and second current pathterminals, and a control terminal, wherein the second current pathterminal of the fourth transistor is coupled to the output terminal,wherein the first current path terminal of the fourth transistor iscoupled to the first current path terminal of the first transistor, andwherein the control terminal of the fourth transistor is coupled to acontrol terminal of the first transistor.
 7. The amplifier of claim 6,further comprising a fifth transistor having first and second currentpath terminals, and a control terminal, wherein the first current pathterminal of the fifth transistor is coupled to the first current pathterminal of the first transistor, wherein the second current pathterminal the fifth transistor is coupled to the output terminal, andwherein the control terminal of the fifth transistor is coupled to thecontrol terminal of the first transistor.
 8. The amplifier of claim 1,further comprising: a first capacitor coupled between the outputterminal and the second current path terminal of the first transistor;and a second capacitor coupled between the output terminal and thesecond current path terminal of the second transistor.
 9. The amplifierof claim 1, further comprising a fourth transistor having first andsecond current path terminals, and a control terminal, wherein thesecond current path terminal of the fourth transistor is coupled to thesecond current path terminal of the third transistor, and wherein thecontrol terminal of the fourth transistor is coupled to the outputterminal.
 10. The amplifier of claim 9, wherein the control terminal ofthe fourth transistor is coupled to the output terminal via a capacitor.11. The amplifier of claim 9, wherein the first, second, and thirdtransistors are n-type transistors, and the fourth transistor is ap-type transistor.
 12. The amplifier of claim 1, further comprising afourth transistor having first and second current path terminals,wherein the first current path terminal of the fourth transistor iscoupled to the first current path terminal of the first transistor. 13.The amplifier of claim 12, wherein the first current path terminal ofthe fourth transistor is coupled to the first current path terminal ofthe first transistor via a first capacitor.
 14. The amplifier of claim13, wherein the first capacitor is coupled between the first currentpath terminal of the fourth transistor and the output terminal, theamplifier further comprising a second capacitor coupled between theoutput terminal and the first current path terminal of the firsttransistor.
 15. The amplifier of claim 1, further comprising: a fourthtransistor having first and second current path terminals, and a controlterminal; a fifth transistor having first and second current pathterminals, and a control terminal, wherein the control terminal of thefifth transistor is coupled to a control terminal of the firsttransistor, and wherein the first current path terminal of the fifthtransistor is coupled to the second current path terminal of the fourthtransistor; and a sixth transistor having first and second current pathterminals, and a control terminal, wherein the first current pathterminal of the sixth transistor is coupled to the first current pathterminal of the fourth transistor, and wherein the second current pathterminal of the sixth transistor is coupled to the first current pathterminal of the first transistor.
 16. The amplifier of claim 15, whereinthe first, second, third, and fifth transistors are transistors of afirst type, and wherein the fourth and sixth transistors are transistorsof a second type that is different from the first type.
 17. Theamplifier of claim 1, wherein the first current path terminal of thefirst transistor is a source terminal, and wherein the second currentpath terminal of the first transistor is a drain terminal.
 18. Theamplifier of claim 1, wherein the amplifier is a class-AB amplifier. 19.The amplifier of claim 1, wherein the reference terminal is a groundterminal.
 20. The amplifier of claim 1, wherein the resistor isconfigured to compensate for an effect of a first parasitic capacitancebetween the first and second current path terminals of the thirdtransistor of a frequency response of the amplifier.